Description of a chromosomal fosA6 tandem multicopy fragment and its in vivo evolution in consecutive KPC-producing Klebsiella pneumoniae clinical isolates.

Chromosomal tandem duplications are scarcely reported in Klebsiella pneumoniae. We describe a chromosomal fosA6 tandem multicopy fragment present at varying copy number in five consecutive K. pneumoniae carbapenemase-producing K. pneumoniae isolates recovered from a single patient over 27 months. Fosfomycin minimum inhibitory concentrations increased with copy number; however, fosA6 expression did not scale proportionally with copy number. No other fosfomycin resistance mechanism was identified. This highlights the clinical relevance of chromosomal AMR gene amplifications.